This invention relates generally to the transmission of microwave and millimeter-wave energy. Specifically, the present invention relates to an apparatus and method for providing a resistor which is useful in stripline transmission systems. More specifically, the present invention relates to fabricating a resistor on a printed circuit board using only conventional printed circuit board fabrication techniques.
A feed network for an antenna may contain hundreds of power dividers or other multiport devices, each of which requires a resistive element. Several conventional techniques teach constructing or coupling resistive elements in transmission systems. Although these techniques may work acceptably when only a few resistors are needed, they become impractical in large feed networks which require hundreds of these resistors.
For example, chip resistors or resistive films may reside at desired locations in the transmission system. These chip resistors or resistive films must be accurately placed and soldered or bonded to a printed circuit board to properly connect the resistive element to the transmission system. However, even slight displacements, such as 0.005 inch, from an optimum position cause a degraded performance in the transmission system at higher frequencies, such as greater than 10 GHz. Thus, when hundreds of these resistors are placed in a transmission system, the probability of degraded performance from a few misplaced resistors becomes great. Furthermore, the inductance of solder joints and the effects of adhesives often degrade the peformance of the transmission system, and the chip resistors may experience fracturing problems when several layers of printed circuit boards are bonded together to form a multi-layer structure.
Thick film resistors might also reside at desired locations on a printed circuit board. A silk screening operation typically deposits these thick film resistors, and the thick film resistors then cure at elevated temperatures. However, when printed circuit boards become relatively large, the silk screening technique fails to accurately place the resistive film. Additionally, the elevated temperatures used to cure the resistive film may degrade the adhesion of printed circuit traces to a dielectric substrate. Further, the thick film substance, when cured, represented a brittle bump which tends to crack when multiple printed circuit boards are bonded together.
Alternatively, thin film resistors may reside at desired locations of a transmission system. However, the thin film resistors require a hard substrate, such as a ceramic or quartz. Hard substrates are not practical for use with large feed networks because large feed networks require more area than is mechanically obtainable with a hard substrate. Thus, the thin film resistor technique does not adequately work for large feed networks.
A bi-metallic clad substrate may serve as a printed circuit board on which the transmission system is formed. Bi-metallic clad substrates have a thin resistive metallic layer clad to a dielectric substrate and a conductive metallic layer clad to the resistive layer. A conventional use of a bi-metallic clad substrate first removes portions of the conductive metallic layer leaving a desired trace pattern of the feed network. Then, the resistive layer is etched so that resistive material remains clad to the substrate at desired locations. Unfortunately, additional portions of the resistive layer remain sandwiched between the etched conductive metallic traces and the dielectric substrate. This additional portion of the resistive layer causes significant transmission losses at higher frequencies, such as those above 10 GHz.